Attenuators are two-port devices used to reduce the amplitude of an input signal, ideally without reflections. Such attenuators may be fixed or variable and differ substantially one from each other depending on the technology adopted.
In Roberto Sorrentino, Giovanni Bianchi, Microwave and RF Engineering, 2010, John Wiley & Sons, Ltd, pp. 190-192, structures and operating principles of passive broadband attenuators are described.
In Roberto Sorrentino, Giovanni Bianchi, Microwave and RF Engineering, 2010, John Wiley & Sons, Ltd, pp. 359-400, it is further shown how it is possible to design variable (voltage or current control) broadband attenuators by replacing the resistors of fixed attenuators with semiconductor devices, such as field-effect transistors (PET) or PIN diodes. Such variable attenuators present a variable attenuation, which depends on some control variables. In principle, a continuously variable attenuator is nothing more than a fixed attenuator (e.g., in a T or II configuration) where controlled variable resistors replace fixed ones. In the following, an example FET implementation of a conventional continuous-variable attenuator will be described.
FIG. 3 shows a schematic diagram of a resistive attenuator structure 300 according to the prior art. The conventional resistive attenuator structure 300 comprises as source 310, a load 320 and a resistive attenuator 330 in a T configuration. The resistive attenuator 330 in the T configuration can also be referred to as a resistive T attenuator. The source 310 of the resistive attenuator structure 300 may comprise a voltage generator 312 for generating a source voltage (e.g., VIN=2V) and source resistance 314 (e.g., RS=50Ω). As shown in FIG. 3, the source 310 may be configured for providing an RF input voltage (V1) at a first port 30 (input port). In addition, the load 320 of the resistive attenuator structure 300 may be coupled to a second port 302 (output port). The load 320 may comprise a load resistance 322 (e.g. RL=50Ω). At the output port 302, there will be an RF output voltage (V3). Referring to the schematic diagram of FIG. 3, the resistive T attenuator 330 is connected between the first port 301 or input port and the second port 302 or output port. The resistive T attenuator 330 may comprise a first series resistance 332-1, R1, a second series resistance 332-2, R3, and a shunt resistance 334, R2. The first port 301 or input port is connected to the first series resistance 332-1, while the second port 302 or output port is connected to the second series resistance 332-2. Moreover, the first and the second series resistances 332-1, 332-2 are connected in series between the first and the second ports 301, 302, As shown in FIG. 3, the shunt resistance 334 is connected between the first and the second series resistances 332-1, 332-2. The shunt resistance 334 is coupled to an intermediate node 303 (voltage V2).
The conventional resistive attenuator structure 300 of FIG. 3 can be characterized by the following synthesis formulae:
                    R        1            ⁡              (                  A          dB                )              =                            R          3                ⁡                  (                      A            dB                    )                    =                        R          0                ⁢                              1            -                          10                                                A                  dB                                20                                                          1            +                          10                                                A                  dB                                20                                                                            R        2            ⁡              (                  A          dB                )              =                  R        0            ⁢                        2          ·                      10                          -                                                A                  dB                                20                                                              1          -                      10                          -                                                A                  dB                                20                                                        
Here, R0 and AdB denote the working or reference impedance (50Ω in most cases) and a predefined attenuation in dB of the resistive T attenuator, respectively.
In the following, the voltages V2, V3 at the different nodes (at node 303 and at the output port 302) are given for an example unitary RF input voltage. This means that in FIG. 3, V1=1 at the input port 301. Those voltages V2, V3 given rough estimation of the compression. Note that a voltage source amplitude of 2V gives V1=1 if the attenuator is impedance matched, as it is if its component values are given by the synthesis formulae (see equations above). The corresponding equations read:
                    V        2            ⁡              (                  A          dB                )              =                                        [                                                            R                  3                                ⁡                                  (                                      A                    dB                                    )                                            +                              R                0                                      ]                    ·                                    R              2                        ⁡                          (                              A                dB                            )                                                                          R              3                        ⁡                          (                              A                dB                            )                                +                      R            0                    +                                    R              2                        ⁡                          (                              A                dB                            )                                                                                      [                                                                    R                    3                                    ⁡                                      (                                          A                      dB                                        )                                                  +                                  R                  0                                            ]                        ·                                          R                2                            ⁡                              (                                  A                  dB                                )                                                                                        R                3                            ⁡                              (                                  A                  dB                                )                                      +                          R              0                        +                                          R                2                            ⁡                              (                                  A                  dB                                )                                                    +                              R            1                    ⁡                      (                          A              dB                        )                                                  V        3            ⁡              (                  A          dB                )              =                            V          3                ⁡                  (                      A            dB                    )                    ⁢                        R          0                                      R            0                    +                                    R              3                        ⁡                          (                              A                dB                            )                                                              ∂                  [                                                    V                3                            ⁡                              (                                  A                  dB                                )                                      -                                          V                2                            ⁡                              (                                  A                  dB                                )                                              ]                            ∂                  A          dB                      =                            ln          ⁡                      (            10            )                          20            ⁢                                    10                          -                                                A                  dB                                20                                              -                      10                                          A                dB                            20                                +          2                                      (                          1              +                              10                                                      A                    dB                                    20                                                      )                    2                    
FIG. 4 shows a graph 400 of example voltages across the attenuator's resistors (402, 404, 406) in dependence on the attenuation in dB (ATT_dBk) for the circuit of FIG. 3 when the input RF voltage amplitude is unitary (V1=1). In the graph 400 of FIG. 4, the y-axis (ordinate) refers to the voltage 420 in V, while the x-axis (abscissa) refers to the attenuation 410 in dB. Note that the maximum voltage across R1 (the first series resistance 332-1 of the resistive attenuator 330 in FIGS. 3) and R2 (the shunt resistance 334 of the resistive attenuator 330 in FIG. 3) is unitary, while the maximum amplitude across R3 (the second series resistance 332-2 of the resistive attenuator 330 in FIG. 3) is 0.173, which is much smaller. Therefore, from this point, we will focus our attention on the resistances R1, R2. More precisely, that peak 408 (which corresponds to a maximum of the curve 406 for the voltage across the resistance R3) is achieved for an attenuation of 20log10(1+0.50.5)=7.656 dB and equals to (1+0.50.5).2=0.173. In the following, the sensitivity of the attenuation to the component values will be derived.
The attenuation (α or V3/V1) in linear units with its derivatives in respect of the various resistances is given by the following equations:
                    V        ⁢                                  ⁢        3                    V        ⁢                                  ⁢        1              =                            R          ⁢                                          ⁢          2                          R          0                                                                R              ⁢                                                          ⁢              2                                      R              0                                ·                      (                          1              +                                                R                  ⁢                                                                          ⁢                  1                                                  R                  0                                            +                                                R                  ⁢                                                                          ⁢                  3                                                  R                  0                                                      )                          +                                            R              ⁢                                                          ⁢              1                                      R              0                                ·                      (                          1              +                                                R                  ⁢                                                                          ⁢                  3                                                  R                  0                                                      )                                    α    =                                        R            ⁢                                                  ⁢            2                                R            0                                                                              R                ⁢                                                                  ⁢                2                                            R                0                                      ·                          (                              1                +                                                      R                    ⁢                                                                                  ⁢                    1                                                        R                    0                                                  +                                                      R                    ⁢                                                                                  ⁢                    3                                                        R                    0                                                              )                                +                                                    R                ⁢                                                                  ⁢                1                                            R                0                                      ·                          (                              1                +                                                      R                    ⁢                                                                                  ⁢                    3                                                        R                    0                                                              )                                          ⁢                          [                                                                                    ⅆ                                                            ⅆ                      R                                        ⁢                                                                                  ⁢                    1                                                  ⁢                α                            =                                                                                          R                      ⁢                                                                                          ⁢                      2                                                              R                      0                                                        ·                                      (                                          1                      +                                                                        R                          ⁢                                                                                                          ⁢                          2                                                                          R                          0                                                                    +                                                                        R                          ⁢                                                                                                          ⁢                          3                                                                          R                          0                                                                                      )                                    ·                                      1                                          R                      0                                                                                                            [                                                                                                                        R                            ⁢                                                                                                                  ⁢                            2                                                                                R                            0                                                                          ·                                                  (                                                      1                            +                                                                                          R                                ⁢                                                                                                                                  ⁢                                1                                                                                            R                                0                                                                                      +                                                                                          R                                ⁢                                                                                                                                  ⁢                                3                                                                                            R                                0                                                                                                              )                                                                    +                                                                                                    R                            ⁢                                                                                                                  ⁢                            1                                                                                R                            0                                                                          ·                                                  (                                                      1                            +                                                                                          R                                ⁢                                                                                                                                  ⁢                                3                                                                                            R                                0                                                                                                              )                                                                                      ]                                    2                                                                                                                                          ⅆ                                                            ⅆ                      R                                        ⁢                                                                                  ⁢                    2                                                  ⁢                α                            =                                                                    (                                          1                      +                                                                        R                          ⁢                                                                                                          ⁢                          3                                                                          R                          0                                                                                      )                                    ·                                                            R                      ⁢                                                                                          ⁢                      1                                                              R                      0                                                        ·                                      1                                          R                      0                                                                                                            [                                                                                                                        R                            ⁢                                                                                                                  ⁢                            2                                                                                R                            0                                                                          ·                                                  (                                                      1                            +                                                                                          R                                ⁢                                                                                                                                  ⁢                                1                                                                                            R                                0                                                                                      +                                                                                          R                                ⁢                                                                                                                                  ⁢                                3                                                                                            R                                0                                                                                                              )                                                                    +                                                                                                    R                            ⁢                                                                                                                  ⁢                            1                                                                                R                            0                                                                          ·                                                  (                                                      1                            +                                                                                          R                                ⁢                                                                                                                                  ⁢                                3                                                                                            R                                0                                                                                                              )                                                                                      ]                                    2                                                                                                                                          ⅆ                                                            ⅆ                      R                                        ⁢                                                                                  ⁢                    3                                                  ⁢                α                            =                                                                                          R                      ⁢                                                                                          ⁢                      2                                                              R                      0                                                        ·                                      (                                                                                            R                          ⁢                                                                                                          ⁢                          2                                                                          R                          0                                                                    +                                                                        R                          ⁢                                                                                                          ⁢                          1                                                                          R                          0                                                                                      )                                    ·                                      1                                          R                      0                                                                                                            [                                                                                                                        R                            ⁢                                                                                                                  ⁢                            2                                                                                R                            0                                                                          ⁢                                                  (                                                      1                            +                                                                                          R                                ⁢                                                                                                                                  ⁢                                1                                                                                            R                                0                                                                                      +                                                                                          R                                ⁢                                                                                                                                  ⁢                                3                                                                                            R                                0                                                                                                              )                                                                    +                                                                                                    R                            ⁢                                                                                                                  ⁢                            1                                                                                R                            0                                                                          ·                                                  (                                                      1                            +                                                                                          R                                ⁢                                                                                                                                  ⁢                                3                                                                                            R                                0                                                                                                              )                                                                                      ]                                    2                                                                        ]      
Subsequently, the FET implementation of the continuous-variable attenuator is described with reference to FIGS 5 and 6. FIG. 5 shows a schematic diagram of a conventional continuous-variable FET attenuator 530 within the FET implementation 500. In the FET implementation 500 of FIG. 5, a source 510 comprising a voltage generator 512 and a source resistance 514, a load 520 comprising a load resistance 522 and the variable FET attenuator 530 are shown. Here, the source 510 with the circuit elements 512, 514 and the load 520 with the load resistance 522 as shown in the FET implementation 500 of FIG. 5 essentially correspond to the source 310 with the circuit elements 312, 314 and the load 320 with the load resistance 322 as shown in the resistive attenuator structure 300 of FIG. 3. Correspondingly, the source 510 is configured for providing an RF input voltage V1 at a first port 501, while the load 520 is coupled to a second port 502. The variable FET attenuator 530 is connected between the first and the second ports 501, 502.
The variable FET attenuator 530 of FIG. 5 comprises a first series resistance 532-1, a second series resistance 532-2 and an adjustable shunt resistance 534. As shown in FIG. 5, the first and the second series resistances 532-1, 532-2 may be configured to be adjustable. The first and the second adjustable series resistances 532-1, 532-2 and the adjustable shunt resistance 534 may be implemented by using controllable field street transistors (FET).
The control of the adjustable resistances of the FET can be achieved by providing a bias network 535. The FET implementation 500 of FIG. 5 essentially represents a limited maximum attenuation structure.
Here, the first and the second series resistances 532-1, 532-2 and the adjustable shunt resistance 534 of the variable FET attenuator 530 in FIG. 5 essentially correspond to the first and the second series resistances 332-1, R1, 332-2, R3, and the shunt resistance 334. R2, of the resistive T attenuator 330 in FIG. 3. However, the first and the second series resistances 532-1, 532-2 and the shunt resistance 534 of FIG. 5 represent adjustable resistances, while the first and the second series resistances 332-1, 332-2 and the shunt resistance 334 of FIG. 3 represent fixed resistances.
Referring to the implementation of FIG. 5, each of the first and the second series resistances 532-1, 532-2 of the variable FET attenuator 530 comprises a parallel circuit of a fixed resistor 536-1. R1B; 536-2, R3B, and an FET element 538-1, Q1; 538-2, Q3.
In the following considerations, the drain-source resistance of the FET Q1, Q3 (Q2: FET element associated with the adjustable shunt resistance 534) will be denoted as R1A=Rds(Q1), R3A=Rds(Q3) [R2=Rds(Q2)]. Hence, R1=R1A/R1B and R3=R3A/R3B.
The resistances R1B=R3B assume the value of R1=R3 corresponding to the maximum attenuation (in the maximum attenuation state, Q1 and Q3 are completely pinched-off). Namely, if the maximum attenuation in dB is 1, 2, 3, 5, 6, 10, 20, 30, or 40 dB, the resulting resistances (in the case of R0=50Ω) R1B=R3B become 2.875. 5.731, 8.55, 14.006, 16.614, 25.975, 40.909, 46.935, or 49.01, respectively.
      R    ⁢                  ⁢    1    ⁢    B    =      [                            1                          2.875                                      2                          5.731                                      3                          8.55                                      5                          14.006                                      6                          16.614                                      10                          25.975                                      20                          40.909                                      30                          46.935                                      40                          49.01                      ]  
The above table gives the maximum series resistance (i.e., R1B=R3B) for different values of the maximum attenuation (the two quantities are reciprocally monotonic). On the right hand side of that equation, the first column describes the maximum attenuation in dB, while the second column describes R1B=R3B.
Referring to the implementation of FIG. 5, it is important to note that the fixed resistors 536-1, 536-2, R1B=R3B are in shunt between the drain-source of the series transistors 538-1, 538-2 (Q1, Q3). That reduces the sensitivity of the global series resistances 532-1, 532-2 (R1, R3) to the drain-source resistance (R1A, R3A) of the transistors.
Indeed, let α be the transmission coefficient amplitude in linear units, then its derivative respect of the series input resistance is
            ∂              [                  α          ⁡                      (                                          R                1                            ,                              R                2                            ,                              R                3                                      )                          ]                    ∂              R        1              =      -                                        R            2                                R            0                          ·                  (                      1            +                                          R                2                                            R                0                                      +                                          R                3                                            R                0                                              )                ·                  1                      R            0                                                [                                                                      R                  2                                                  R                  0                                            ·                              (                                  1                  +                                                            R                      2                                                              R                      0                                                        +                                                            R                      3                                                              R                      0                                                                      )                                      +                                                            R                  1                                                  R                  0                                            ·                              (                                  1                  +                                                            R                      3                                                              R                      0                                                                      )                                              ]                2            
The resulting input series resistance is R1=R1A/R1B.
The derivative of α in respect of the drain-source resistance (R1A) of Q1 can be found by applying the derivation rule for composite functions, which gives
            ∂              [                  α          ⁡                      (                                                            R                  1                                =                                                                            R                                              1                        ⁢                        A                                                              ·                                          R                                              1                        ⁢                        B                                                                                                                        R                                              1                        ⁢                        A                                                              +                                          R                                              1                        ⁢                        B                                                                                                        ,                              R                2                            ,                              R                3                                      )                          ]                    ∂              R                  1          ⁢          A                      =                    ∂                  [                      α            ⁡                          (                                                R                  1                                ,                                  R                  2                                ,                                  R                  3                                            )                                ]                            ∂                  R          1                      ⁢                  ∂                  (                                                    R                                  1                  ⁢                  A                                            ·                              R                                  1                  ⁢                  B                                                                                    R                                  1                  ⁢                  A                                            +                              R                                  1                  ⁢                  B                                                              )                            ∂                  R                      1            ⁢            A                              
The following description relates to a numerical computation of the derivative of the transmission coefficient amplitude in respect of the drain-source resistance (R1A) of Q1, as a function of the set attenuation and for different values of set and maximum attenuation. This numerical computation gives:
      R    ⁢                  ⁢    1    ⁢    B_Att    ⁢    _Max    ⁢    _dB    =      [                            5                          14.006                                      10                          25.975                                      15                          34.902                                      20                          40.909                                      25                          44.676                                      30                          46.935                      ]  
On the right hand side of the above equation, the first column describes the maximum attenuation in dB, while the second column describes the fixed series resistance R1B, R3B. Moreover, FIG. 6 shows a graph 600 of derivatives 620 of the transmission coefficient amplitude (α) in respect of the drain-source resistance (R1A) of Q1, as a function of the set attenuation 610 (ATT_dBk), for different values (611, 612, 613, 614, 615) of set and maximum attenuation. In the graph 600 of FIG. 6, the dashed curve 605 indicated by “No R1B” refers to the case of the absence of the fixed series resistance R1B.
Note that the presence of the fixed series resistance R1B has no mitigation effect at zero attenuation, as expected, since R1A=0 in that state.
Nevertheless, any practical attenuator of the type in FIG. 5 has a nonzero minimum attenuation (insertion-loss), which typically falls between 1 and 2 dB. At that minimum insertion-loss, the sensitivity reduction of the transmission coefficient to R1A ranges from 30% to 80% if the maximum attenuation is 5 dB and if the minimum ranges between 1 and 3 dB, as the following computations show:
      GR    ⁢                  ⁢    1    =      [                            5                          0.702                          0.424                          0.198                                      10                          0.879                          0.737                          0.587                                      15                          0.936                          0.848                          0.743                                      20                          0.961                          0.897                          0.816                                      25                          0.973                          0.922                          0.852                                      30                          0.979                          0.935                          0872                      ]  
The above equation gives the sensitivity reduction (improvement) factor of the linear attenuation to the input FET drain-source resistance. In particular, on the right hand side of that equation, the first column indicates the maximum attenuation in dB, while the second, third and fourth columns show the values for a minimum attenuation of 1, 2 and 3 dB respectively.
However, it has been found that the conventional variable attenuators including the FET implementation of the continuous-variable attenuator according to FIG. 5 present some problems.
Firstly, it has been found that the resistance that the used semiconductor devices present in RF is not only a function of the applied control quantity (voltage for FET or current for PIN diodes). Rather, such resistance is also depending from the RF voltage applied to the device itself. As a consequence, the attenuation is not constant with the applied RF power.
Moreover, the classical well-known nonlinear (undesired) effects affect the variable attenuator. These are power compression, harmonic (with single-tone excitation) and intermodulation (with multiple-tone excitation) distortion.
Secondly, it has been found that the precision of the actuated attenuation value depends on the precision of the applied control quantity. In some cases and/or regions of the attenuation value, the sensitivity of the latter to the control quantity value could be quite high. A typical case is with FET devices at intermediate (between minimum and maximum) attenuation values, when the FET operate close to their pinch-off region (although not completely pinched-off).
Thirdly, it has been found that semiconductor devices sometimes present long-term settling time effects when abrupt changes are applied on the control quantity. Thus, the resulting RF equivalent resistance reaches a value close to the final one within a short time (in the order of nanoseconds), but the residual change needs a long time (up to seconds) to set. Consequently, the RF attenuation response to the control quantity could be very slow, particularly if precise and accurate attenuation values are required.
Furthermore, it must be noted that in a maximum attenuation limited attenuator, such as in the conventional continuous-variable FET attenuator of FIG. 5, the series resistance has a maximum value smaller than the working resistance (R0). In the previous description, it has been shown that this helps in keeping the attenuation variations less sensitive to the FET channel resistance, which is inherently nonlinear and depends on the applied input power. However, it has been found that also in the case of the maximum attenuation limited attenuator, it is sometimes difficult to avoid the above mentioned problems, especially for a good realization in practical cases.
Therefore, it is an object of the present invention to provide a concept of a variable attenuator which allows a better characteristic of an attenuation in a real implementation.
This object is achieved by a variable attenuator according to claim 1 or a resistive attenuator structure according to claim 12.
The basic idea underlying the present invention is that the above-mentioned concept of the variable attenuator can be achieved if a series resistance and an adjustable shunt resistance are provided, wherein the adjustable shunt resistance comprises a series circuit of a fixed resistor and a semiconductor element having an adjustable resistance. This allows to obtain the better attenuation characteristic in the context of a real implementation of the variable attenuator.
According to an embodiment of the present invention, a variable attenuator comprises a series resistance and an adjustable shunt resistance. The adjustable shunt resistance comprises a series circuit of a fixed resistor and a semiconductor element having an adjustable resistance.
According to a further embodiment of the present invention, the semiconductor element of the series circuit of the adjustable shunt resistance is a field-effect transistor (FET). The field-effect transistor is configured such that, for a minimum attenuation value in a range of 1 to 3 dB, a voltage applied between a drain and a source of the field-effect transistor is reduced by a voltage reduction value in a range of 10 to 60% as compared to a voltage applied between the drain and the source of the field-effect transistor alone. By such a voltage reduction, some of the undesired nonlinear effects originating from the field-effect transistor can be efficiently reduced. Therefore, an improved quality of the attenuation provided with the variable attenuator can be achieved.
According to a further embodiment of the present invention, a resistive attenuator structure comprises a source, a load and an inventive variable attenuator. The source is configured for providing an RF input voltage at a first port. The load is coupled to a second port. The inventive variable attenuator is connected between the first port and the second port.